Hitherto, iron-based powder compositions for powder metallurgy have been produced by a mixing method in which alloying powders such as core materials such copper, graphite, and iron phosphide powders, are mixed with an iron powder, and according to the necessity, in addition to the powders for improving the machinability, a lubricant such as zinc stearate, aluminium stearate, and lead stearate is mixed into the composition. Such a lubricant has been adopted in view of a desire for homogeneous mixing with a metal powder, easy decomposition and removability of materials other then the core materials at the time of sintering.
Recently, as the requirement of higher strength for sintering manufactures is increased, as disclosed in U.S. Pat. No. 5,256,185 and U.S. Pat. No. 5,368,630, it has been proposed in the industry to use a warm compaction technology which permits higher density and higher strength of compacts by means of performing a compaction while metal powders are heated. It is considered for the lubricant used in such a compaction procedure that a lubricity at the time of heating is important as well as the homogeneous mixing with a metal powder, the ease of decomposition and the removability of the lubricant at the time of sintering. Specifically, a mixing of mixtures of a plurality of lubricants having mutually different melting points with metal powders serves, at the time of a warm compaction, to melt part of the lubricants, to uniformly spread the lubricants between iron and/or alloying metal particles, and to decrease frictional resistances among the particles and between a compacted form and dies, so that compactibility of the material and accuracy of the formed rendition is improved.
However, such a metal powder composition involves the following drawbacks. First, a raw material mixture undergoes segregation. Regarding the segregation, since the metal powder composition contains powders having different sizes, shapes and densities, segregation occurs readily during transport after mixing and upon charging the powder composition into hoppers, or upon discharging the powder composition from the hoppers or during molding treatments. For example, it is well known that segregation of a mixture of iron-based powder and graphite powder occurs within a transport vehicle owing to vibrations during trucking, so that the graphite powder rises to the top. It is also known with respect to graphite charged into a hopper that the concentration of graphite powder differs at the beginning, middle, and end of the discharging operation from the hopper owing to segregation within the hopper. These segregations cause fluctuations in the composition of products of the powder metallurgy; fluctuations in dimensional changes and strength become large, and this causes the production of inferior products.
The flow rate of the powder composition increases as a result of the increased specific surface area of the mixture, since graphite and other powders are fine powders. Such increases in flow rate are disadvantageous because it decreases the production speed of green compacts by decreasing charging speed of the powder composition into die cavities for compaction.
As for technologies for preventing segregation of such a powder composition, there are known methods based on selection of an appropriate binder as disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Sho.56-136901 and Japanese Patent Application Laid Open Gazette (Kokai) Sho.58-28321. However, these methods involve such a drawback that if the quantity of binder added is increased so that segregation of the powder composition is sufficiently improved, the flow rate of the powder composition is increased.
Japanese Patent Application Laid Open Gazette (Kokai) Hei.1-165701 and Japanese Patent Application Laid Open Gazette (Kokai) Hei.2-47201 proposed methods in which a melt composed of the combination of an oil and a metal soap or wax, melted together is selected as a binder. These methods make it possible to sufficiently reduce segregation of a powder composition and dust generation, and also to improve the flowability. However, these methods involve such a problem that the flowability of the powder composition varies with the passage of time owing to means for preventing the segregation mentioned above. Previous technologists developed a method in which a melt composed of the combination of a high-melting point of oil and a metal soap, melted together is selected as a binder, as proposed in Japanese Patent Application Laid Open Gazette (Kokai) Hei.2-57602. According to this method, the melt has a small change of elapse, and a change of elapse of flow rate of the powder composition is reduced. However, this method involves another drawback such that apparent density of the powder composition varies, since a high-melting point of saturated fatty acid of solid state and a metal soap are mixed with iron-based powders at the room temperature.
U.S. Pat. No. 5,989,304 describes an iron-based powder composition for powder metallurgy excellent in flowability and compactibility is produced in accordance with a method comprising the steps of: adding to iron-based and alloying powders, for a primary mixing, a surface treatment agent, and in addition, for a secondary mixing, a fatty acid amide and at least one lubricant, wherein the lubricant has a melting point higher than that of the fatty acid amide and can be a thermoplastic resin, a thermoplastic elastomer, and inorganic or organic compounds having a layered crystal structure; heating and stirring up a mixture after the secondary mixing at a temperature above a melting point of the fatty acid amide to melt the fatty acid amide; cooling, while mixing, the mixture subjected to the heating and stirring process so that the alloying powder and a lubricant having a melting point higher than the fatty acid amide adhere to a surface of the iron base powder subjected to the surface treatment by an adhesive force of the melt; and adding at the time of the cooling, for a tertiary mixing, a metallic soap and at least one thermoplastic resin or thermoplastic elastomer powders and inorganic or organic compounds having layered crystal structure. The mixture is heated to about 423K and loaded into a die for compaction.
U.S. Pat. No. 6,139,600 describes an iron-based powder composition made by a process comprising the steps of: adding to iron-based and alloying powders, for a primary mixing, a surface treatment agent, and in addition, for a secondary mixing, a fatty acid amide and at least one lubricant, wherein the lubricant has a melting point higher than that of the fatty acid amide and can be, a thermoplastic resin, a thermoplastic elastomer, and inorganic or organic compounds having a layered crystal structure; heating and stirring up a mixture after the secondary mixing at a temperature above a melting point of the fatty acid amide to melt the fatty acid amide; cooling, while mixing, the mixture subjected to the heating and stirring process so that the alloying powder and a lubricant having a melting point higher than the fatty acid amide adhere to a surface of the iron base powder subjected to the surface treatment by an adhesive force of the melt; and adding at the time of the cooling, for a tertiary mixing, a metallic soap and at least one a thermoplastic resin or thermoplastic elastomer powders and inorganic or organic compounds having layered crystal structure. The mixture is heated to about 423K and loaded into a die for compaction.
U.S. Pat. No. 6,235,076 describes an iron-based powder composition for powder metallurgy having excellent flowability at room temperature and a warm compaction temperature, having improved compactibility enabling lowering ejection force in compaction, to provide a process for producing the iron-based powder composition, and to provide a process for producing a compact of a high density from the iron-based powder composition. The iron-based powder composition comprises an iron-based powder, a lubricant, and an alloying powder, and at least one of the iron-based powder, the lubricant, and the alloying powder is coated with at least one surface treatment agent selected from the group of surface treatment agents of organoalkoxysilanes, organosilazanes, titanate coupling agents, fluorine-containing silicon silane coupling agents. The iron-based powder composition is compacted at a temperature not lower than the lowest melting point of the employed lubricants, but not higher than the highest melting point of the employed lubricants.
U.S. Pat. No. 5,756,788 discloses a process for making metal oxide hydrophobic by coating the metal oxide with a silicone polymer is disclosed. The hydrophobic metal oxide is prepared by contacting the metal oxide with a reactive silicone compound and then in a subsequent step the coated metal oxide is heated to 40° to 100° C. for between 1 and 10 hours. The resulting metal oxide is hydrophobic, non-reactive, not affected by water and can be applied to the skin for protection from ultraviolet light of the sun. U.S. Pat. No. 6,045,650 discloses a method for adjusting the surface properties of materials by the application and reaction of liquid compositions. The process applies a solid coating onto a surface of an article. The surface of an article has a first physical property measurable as a degree of hydrophobicity and/or hydrophilicity. A liquid coating of an oxidizable material containing at least one element other than carbon, hydrogen, oxygen and nitrogen is applied onto the surface of the article. The oxidizable material is oxidized on the surface to attach an oxidized material having said at least one element other than carbon, oxygen, nitrogen and hydrogen onto said surface. This process thereby changes the first physical property with respect to its hydrophobicity and/or hydrophilicity. The process is relatively gentle to the underlying surface, at least in part because of the moderate temperatures which may be used for oxidation, and a wide range of properties may be provided onto the surface by appropriate selection and/or mixing of the liquid material and selection of the surface. The process is particularly useful with particulate materials.